Georg Dechant

Binding of brain-derived neurotrophic factor to the nerve growth factor receptor

The neurotrophic proteins BDNF and NGF are related in their primary structures, and both have high- and low-affinity receptors on their responsive neurons. In this study, we investigate the extent to which these receptors can discriminate between BDNF and NGF. We found that a 1000-fold excess of the heterologous ligand is needed to reduce binding to the high-affinity receptor by 50%, but that the same concentrations of BDNF and NGF similarly reduce the binding of either ligand to the low-affinity receptor. Results obtained with cells transfected with the low-affinity NGF receptor gene indicate that these cells bind BDNF, in addition to NGF, whereas cells before transfection do not. These data indicate that the low-affinity NGF receptor is also a low-affinity BDNF receptor and that whatever is conferring high-affinity binding and biological response also considerably reinforces the ability of the low-affinity receptor to discriminate between NGF and BDNF.

The neurotrophin receptor p75(NTR) : novel functions and implications for diseases of the nervous system

Neurotrophins have long been known to promote the survival and differentiation of vertebrate neurons. However, these growth factors can also induce cell death through the p75 neurotrophin receptor (p75NTR), a member of the tumor necrosis factor receptor superfamily. Consistent with a function in controlling the survival and process formation of neurons, p75NTR is mainly expressed during early neuronal development. In the adult, p75NTR is re-expressed in various pathological conditions, including epilepsy, axotomy and neurodegeneration. Potentially toxic peptides, including the amyloid β- (Aβ-) peptide that accumulates in Alzheimers disease, are ligands for p75NTR. Recent work also implicates p75NTR in the regulation of both synaptic transmission and axonal elongation. It associates with the Nogo receptor, a binding protein for axonal growth inhibitors, and appears to be the transducing subunit of this receptor complex.

Binding of neurotrophin-3 to its neuronal receptors and interactions with nerve growth factor and brain-derived neurotrophic factor.

Neurotrophin‐3 (NT‐3) has low‐affinity (Kd = 8 × 10(−10) M), as well as high‐affinity receptors (Kd = 1.8 × 10(−11) M) on embryonic chick sensory neurons, the latter in surprisingly high numbers. Like the structurally related proteins nerve growth factor (NGF) and brain‐derived neurotrophic factor (BDNF), NT‐3 also binds to the low‐affinity NGF receptor, a molecule that we suggest to designate low‐affinity neurotrophin receptor (LANR). NT‐3 dissociates from the LANR much more rapidly than BDNF, and more slowly than NGF. The binding of labelled NT‐3 to the LANR can be reduced by half using a concentration of BDNF corresponding to the Kd of BDNF to the LANR. In contrast, the binding of NT‐3 to its high‐affinity neuronal receptors can only be prevented by BDNF or NGF when used at concentrations several thousand‐fold higher than those corresponding to their Kd to their high‐affinity neuronal receptors. Thus, specific high‐affinity NT‐3 receptors exist on sensory neurons that can readily discriminate between three structurally related ligands. These findings, including the remarkable property of the LANR to bind three related ligands with similar affinity, but different rate constants, are discussed.

Brain-derived Neurotrophic Factor is a Survival Factor for Cultured Rat Cerebellar Granule Neurons and Protects them Against Glutamate-induced Neurotoxicity

We have studied the effects of different neurotrophins on the survival and proliferation of rat cerebellar granule cells in culture. These neurons express trkB and trkC, the putative neuronal receptors for brain‐derived neurotrophic factor (BDNF) and neurotrophin‐3 (NT‐3) respectively. Binding studies using iodinated BDNF and NT‐3 demonstrated that both BDNF and NT‐3 bind to the cerebellar granule neurons with a similar affinity of ˜ 2x10‐9 M. The number of receptors per granule cell was surprisingly high, ∼30x10‐4 and 2x 105 for BDNF and NT‐3, respectively. Both NT‐3 and BDNF elevated c‐fos mRNA in the granule neurons, but only BDNF up‐regulated the mRNA encoding the low‐affinity neurotrophin receptor (p75). In contrast to NT‐3, BDNF acted as a survival factor for the granule neurons. BDNF also induced sprouting of the granule neurons and significantly protected them against neurotoxicity induced by high (1 mM) glutamate concentrations. Cultured granule neurons also expressed low levels of BDNF mRNA which were increased by kainic acid, a glutamate receptor agonist. Thus, BDNF, but not NT‐3, is a survival factor for cultured cerebellar granule neurons and activation of glutamate receptor(s) up‐regulates BDNF expression in these cells.

Neurotrophic cross-talk between the nervous and immune systems: implications for neurological diseases.

Inflammatory reactions in the central nervous system usually are considered detrimental, but recent evidence suggests that they also can be beneficial and even have neuroprotective effects. Intriguingly, immune cells can produce various neurotrophic factors of various molecular families. The concept of “neuroprotective immunity” will have profound consequences for the pathogenesis and treatment of neuroinflammatory diseases such as multiple sclerosis. It also will prove important for neurodegenerative disorders, in which inflammatory reactions often occur. This review focuses on recent findings that immune cells produce brain‐derived neurotrophic factor in multiple sclerosis lesions, whereas neurons and astrocytes express the appropriate tyrosine kinase receptor TrkB. Together with functional evidence for the neuroprotective effects of immune cells, these observations support the concept of “neuroprotective immunity.” We next examine current and future therapeutic strategies for multiple sclerosis and experimental autoimmune encephalomyelitis in light of neuroprotective immunity and finally address the broader implications of this new concept for other neuroinflammatory and neurodegenerative diseases. Ann Neurol 2003

Complete ablation of the neurotrophin receptor p75NTR causes defects both in the nervous and the vascular system.

We identified a protein isoform of the common neurotrophin receptor p75NTR that arises from alternative splicing of exon III in the p75NTR locus. Because this protein is left intact in the previously described p75NTR mutant mouse line, we generated a new p75NTR mutant allele. Mice homozygous for this mutation lack both protein isoforms, display severe nervous system defects and reveal a previously unknown role of p75NTR in the formation of blood vessels.

Signalling through the neurotrophin receptor p75NTR

Activation specific tyrosine kinase receptors by neurotrophins accounts for the longest known biological actions of the neurotrophins, in particular the promotion of neuronal survival. However, recent studies have revealed that nerve growth factor, the neurotrophin regarded as best understood, also activates a signalling pathway by binding to the neurotrophin receptor p75(NTR). This receptor belongs to the tumor necrosis factor receptor family and lacks intrinsic catalytic activity. The p75(NTR) receptor binds all neurotrophins with nanomolar affinity; however, nerve growth factor seems to be uniquely able to activate it, causing the death of trkA-negative neurons during normal development. Thus, nerve growth factor prevents programmed cell death through its receptor TrkA, but promotes it by signalling through p75(NTR).

The p75 Neurotrophin Receptor Negatively Modulates Dendrite Complexity and Spine Density in Hippocampal Neurons

The correlation between functional and structural neuronal plasticity is by now well documented. However, the molecular mechanisms translating patterns of neuronal activity into specific changes in the structure of neurons remain unclear. Neurotrophins can be released in an activity-dependent manner, and they are capable of controlling both neuronal morphology and functional synaptic changes. They are thus attractive molecules to be studied in the context of synaptic plasticity. In the CNS, most of the work so far has focused on the role of BDNF and of its tyrosine kinase B receptor (TrkB), but relatively little is known about the function of the pan-neurotrophin receptor p75NTR. In this study, we show in loss-of-function experiments that postnatal hippocampal pyramidal cells in two mutant lines of p75NTR have a higher spine density and greater dendritic complexity than wild-type (WT) mice. Conversely, in a gain-of-function approach, p75NTR overexpression in WT neurons significantly reduces dendritic complexity, as well as spine density in all dendritic compartments. These results show that p75NTR negatively modulates dendritic morphology in adult hippocampal pyramidal neurons and documents a new case of functional antagonism between Trk and p75NTR signaling.

The zinc finger protein NRIF interacts with the neurotrophin receptor p75NTR and participates in programmed cell death

NRIF (neurotrophin receptor interacting factor) is a ubiquitously expressed zinc finger protein of the Krüppel family which interacts with the neurotrophin receptor p75NTR. The interaction was first detected in yeast and then biochemically confirmed using recombinant GST–NRIF fusions and p75NTR expressed by eukaryotic cells. Transgenic mice carrying a deletion in the exon encoding the p75NTR‐binding domain of NRIF display a phenotype which is strongly dependent upon genetic background. While at the F2 generation there is only limited (20%) embryonic lethality, in a congenic BL6 strain nrif−/− mice cannot survive beyond E12, but are viable and healthy to adulthood in the Sv129 background. The involvement of NRIF in p75NTR/NGF‐mediated developmental cell death was examined in the mouse embryonic neural retina. Disruption of the nrif gene leads to a reduction in cell death which is quantitatively indistinguishable from that observed in p75NTR−/− and ngf−/− mice. These results indicate that NRIF is an intracellular p75NTR‐binding protein transducing cell death signals during development.

Nerve Growth Factor Regulates Expression of the Nerve Growth Factor Receptor Gene in Adult Sensory Neurons

Sensory neurons of the adult rat dorsal root ganglion (DRG) can be maintained in culture in the absence of nerve growth factor (NGF). We have thus used dissociated cultures of these neurons to study effects of NGF on the regulation of expression of mRNA encoding the nerve growth factor receptor (NGF‐R). In the absence of NGF, levels of NGF‐R mRNA remained constant for 7 days in cultures of adult rat DRG neurons. In the presence of NGF, NGF‐R mRNA levels rose two ‐ three‐fold after 2 days, reaching plateau levels (five ‐ six‐fold elevation) after 5 days. This NGF‐induced up‐regulation could be demonstrated even after prior NGF‐deprivation for 3–4 days. NGF had no effect upon NGF‐R mRNA levels in DRG non‐neuronal cells. Epidermal growth factor (EGF), fibroblast growth factor (FGF) and ciliary neurotrophic factor (CNTF) were without effect on NGF‐R mRNA levels, but 8‐bromo‐cAMP decreased NGF‐R mRNA levels by 65% after 2 days. NGF also induced a rapid (30 min) rise in expression of c‐fos in DRG neurons, but not in non‐neuronal cells. Our results suggest that endogenous levels of NGF may regulate the expression of NGF‐R in vivo.